The present invention relates to a control circuit for a switched mode power supply. In particular, the present application relates to a switched mode power supply that includes a frequency modulation controller. Switched-mode power supplies, which include voltage converters such as buck converters, for example, typically include, or are connected to a controller that uses pulse width modulation (PWM) to set the duty cycle of the switches. Generally, a PWM controller provides a PWM signal that is used to drive the switches in order to provide the desired output voltage. While specific configurations may vary based on exactly what type of switched mode power supply is used, PWM control is commonly used in a wide variety of power supplies or converters.
The PWM controller may be implemented as an analog circuit or a digital circuit. However, digital control circuits provide some benefits over analog circuits. Some of these benefits are described in co-pending U.S. patent application Ser. No. 11/068,184 entitled HYBRID DIGITAL-ANALOG SWITCHED POWER SUPPLY, filed Feb. 28, 2005, the entire contents of which are hereby incorporated by reference herein. Thus, it has become increasingly common to use digital control circuitry to control the duty cycle of the switches in a switched mode power supply. However, there are certain problems that arise when digital control is used, particularly in high frequency applications. In particular, digital control techniques are difficult to use when the clock frequency is in the very high frequency (VHF) to ultra high frequency (UHF) range. At these high switching frequencies, the resolution offered by the control circuit is often not precise enough. This is the result, at least in part, of the use of PWM control. Consider for example, a circuit in which a 100 Mhz clock signal is used. In this case, increments to the pulse width are limited by the frequency of the clock signal. A minimum increment amount of 10 ns is available for a circuit utilizing a 100 MHz clock signal, operating at a 1 Mhz operating frequency to convert a 12 volt input voltage into a 1 volt output voltage. Since the pulse width is on the order of 100 ns, thus, the minimum incremental change to the pulse width can be no less than 10%. This resolution is simply not sufficient for many applications. Even if a 100 KHz PWM frequency were utilized, the minimum increment to the pulse width would be no less than 1%, which also may not provide suitable resolution for some applications. In addition, when the difference between the input voltage and output voltage increases, these problems are exacerbated even further.
Accordingly, it would be beneficial to provide a control circuit for controlling a switched mode power supply that avoids the problems mentioned above.